Illumination systems for generating light are commonly known, and the same applies to the use of LEDs as light source in such illumination systems. Therefore, a detailed explanation thereof will be omitted here.
Generally speaking, one may define several operational requirements for an illumination system. An obvious requirement is that the system can be switched ON and OFF. A second requirement is dimmability: it is desirable that the intensity of the light output can be varied. A third requirement is color variability: it is desirable that the color of the light output can be varied.
With respect to color, it is commonly known that colors as perceived by the human eye can be described in a two-dimensional color space. In this space, pure or monochromatic colors, i.e. electromagnetic radiation having one frequency within the visible spectrum, are located on a curved line having two end points, corresponding to the boundaries of the visible spectrum. This curve, together with a straight line connecting said end points, forms the well-known color triangle. Points within this triangle correspond to so-called mixed colors. An important feature of colors is that, when the human eye receives light originating from two light sources with different color points, the human eye does not distinguish two different colors but perceives a mixed color, wherein the color point of this mixed color is located on a straight line connecting the two color points of the two light sources, while the exact position on this line depends on the ratio between the respective light intensities. The overall intensity of the mixed color corresponds to the respective light intensities added together. Thus, it is possible to generate light having a color point corresponding to any desired point of said line with, within limits, any desired intensity. Similarly, with three light sources, it is possible to render any color point within the triangle defined by the three respective color points.
In the field of illumination, there is a general desire to be able to generate light of which the color can be controlled. Depending on the type of application, the desired characteristics of the illumination system may be different. A specific type of illumination system is a daylight lamp capable of generating white light and/or capable of simulating the change in light color of daylight from sunrise to sunset. Another specific type of illumination system is a replacement for an incandescent lamp, having the same “warm” light output.
While the above basically applies to any type of light source, a light source particularly suitable in color systems is the LED, in view of its size and cost, and considering the fact that an LED produces monochromatic light. Thus, illumination systems have been developed comprising 3 or 4 (or even more) different LED types. By way of example, the RGBW system is mentioned, comprising RED, GREEN, BLUE and WHITE LEDs.
In order to be able to achieve dimmability in an LED system, it is known to apply pulse width modulation: instead of a constant current, the LED receives current pulses of a certain duration at a certain repetition frequency, selected to be sufficiently high such as not to lead to perceivable flicker.
For driving an LED, an LED driver is used, capable of generating the required LED current at the corresponding drive voltage.
In order to be able to set and/or vary a desired color point of the light output, it is necessary to be able to individually vary the intensities of the different colors. While a simple system may comprise one LED per color, practical systems usually have a plurality of LEDs per color. It is possible to drive an array of LEDs by one common driver, and the LEDs may be connected in parallel or in series, or both. Nevertheless, the prior art requires that there be at least one driver per color. This makes such a system relatively costly. Further, between driver system and LED system at least 5 wires are needed, even 8 wires if it is undesirable to have a common ground.